Friday, August 3, 2012

Why the Sky is Blue

Today it found out why the sky is blue. For the short answer, when light
from the sun enters our atmosphere it collides with molecules in the air.
The blue part of the light gets scattered more than the other parts during
these collisions and thus makes the sky appear to our eyes as blue. If the
light from the sun took a straight path down to our eyes with no scattering
or absorption in the atmosphere, the sky would in fact look much as it does
at night in the day time.
So a little background. White light waves from the sun are in fact
mixtures of all colors of the light spectrum. Anyone who’s ever had a
prism knows that when white light shines through it, the light gets
separated and you get a rainbow spectrum showing up on the other side.
Humans can only see a portion of the total light spectrum of energy; we
see from Violet, which has a wavelength of about 380 nanometers, to red
which has a wavelength of about 720 nanometers. Descending from red to
violet, we get orange, yellow, green, blue, and indigo between the two.
Another needed piece of information is that the atmosphere of the earth is
made up of almost all nitrogen and oxygen.
So specifically, what is happening here is that because the wavelength of
blue light is about the same size of an atom of oxygen, which makes up 21%
of the earth’s atmosphere, blue light gets scattered by these oxygen
molecules while the reds, oranges, and others more or less pass straight
through the atmosphere without being scattered much at all. So when you
look up at the sky, everywhere you look looks blue as the blue portion of
the light spectrum bounces off an oxygen molecule and into your eye.
It actually turns out that the earth’s atmosphere is also scattering quite
a bit of violet light as well. So why doesn’t the sky look more like a
blueish/violet? This is for a few reasons. Most important is that our
eyes are most sensitive to blue, red, and green. So our eyes are naturally
more inclined to respond to blue over violet. Also important is that the
sun doesn’t put out light at the same intensity at all wavelengths of the
spectrum. In addition to this less amount, some of the violet also gets
absorbed in the upper atmosphere; so not as much of it gets to our eyes.
So it turns out more or less a combination of these three things makes it
so we see the sky as blue instead of violet or blueish/violet. It is
interesting to note however, that our eyes are picking up some of the
violet and indigo being scattered. Without this, we would in fact see the
sky as more of a blueish green. This is because the green cones in our
eyes respond to the small amounts of scattered yellow light wavelengths to
some extent; meanwhile our eye’s red cones respond to the indigo/violet
somewhat which balances things out a bit and so we only perceive the blue
being scattered.
So you might now be asking, “Why is the sky red or orange when the sun is
setting?” What’s going on here is that as the sun is setting, the light
you are seeing from it is having to travel through a lot more atmosphere
given your angle to the sun. Light at lower wavelengths gets scattered
more than at the higher wavelengths. So less of the blue light gets to you
as it’s being scattered more than, for instance, yellow, orange, and red.
In this case, if it is a relatively clear day, the sky will appear more
yellow than blue as you watch the sunset because the blue is being
scattered so much it never reaches you or at least, not as much of it, but
the yellow isn’t scattered nearly as much but still scattered enough to
make the sky look yellow. If it is dusty or there are a lot of other types
of particles in the air along the path to where you are looking at the sun,
the sky will appear more red. If there is a lot of salt in the air, such
as at sea, it will appear more orange.
So, if there was no scattering or absorption, the sky would appear black in
the day time. If there was more absorption or scattering going on than
there is, then the sky might appear to us to be yellow, orange, or red all
day long.